Examples of known applications of nonwoven fabrics include various types of agricultural covering materials such as full-covering sheets for tunnel culture and outdoor culture.
Nonwoven fabrics composed of, for example, polyethylene terephthalate (PET) or polypropylene (PP) exhibit excellent heat retaining properties and permeability and their performance makes them suitable as nonwoven sheets for agricultural use. On the other hand, such nonwoven fabrics are basically composed of hydrophobic resins and thus have a tendency to allow rain water and so forth to collect on their surface because water does not easily pass therethrough. There is therefore a danger of creating a problem whereby crops being grown are crushed by the weight of waterdrops that have collected on the nonwoven fabric used in a certain form.
The inventors of the present invention developed a full-covering material for agricultural use made of a nonwoven fabric composed of hydrophobic fibers of 6 denier or more and having a METSUKE ( weight/m.sup.2)amount of 10 to 30 g/m.sup.2 and light transmittance of 85% or more as described in Japanese Patent Laid-Open No. 128955/1986.This material suffers from the same problem as that described above because it is composed of hydrophobic fibers.
An agricultural sheet composed of a nonwoven fabric which is subjected to hydrophilic treatment with a surfactant for the purpose of preventing water from collecting on its surface is known as a sheet that is capable of resolving the above-described problem. Such a hydrophilic-treated nonwoven fabric is subjected to hydrophilic treatment to a high degree so that any water in contact with the surface of the nonwoven fabric can pass therethrough as fast as possible from the obverse surface of the nonwoven fabric to the underside thereof. As a result, the hydrophobic property of the fibers which form the nonwoven fabric deteriorates and thus waterdrops held in the voids of the nonwoven fabric do not easily escape therefrom. This leads to a situation in which a significant increase in weight occurs in the nonwoven fabric due to the water retention phenomenon.
Although the danger that the growing crops will be crushed due to the increase in weight of the sheet because of water retention, is not as great as it is with the above-described nonwoven sheeet for agricultural use composed of a nonwoven fabric. In any event, is not possible to completely avoid this problem.
In the case of a hydrophobic nonwoven fabric, the water which collects on its surface can be easily removed by inclining the surface to allow it to run off, and thus any increase in the weight caused by this water can be ignored. On the other hand, since conventional hydrophilic-treated nonwoven fabrics exhibit a high percentage of water retention and the water held in the voids of the nonwoven fabric cannot be easily removed, the state of gradual increase in the weight continues for a long time. This leads to the problem that the increase in weight makes the work of removing the sheet very laborious.
When the sheet is rolled up and stored for reuse after it has been used, there is also a great danger that, since the rolled up nonwoven fabric has a form in which both of the roll ends are fixed, the nonwoven fabric is broken owing to shrinkage that occurs because of the drying that takes place during storage if the fabric has a high degree of water retention.